Coin sorting machines are conventionally known which determine whether to accept an inserted coin depending on changes in impedance of a circuit which occur when the inserted coin blocks the magnetic flux developed by a coin sorting coil. Such a coin sorting machine is described in U.S. Pat. No. 3,870,137.
In the machine described in U.S. Pat. No. 3,870,137 coins formed with thin outer layers laminated on an inner, center layer of a different material (laminated coins), such as dimes (10 cents) and quarters (25 cents), are sorted by detecting the two types of materials of the coin, i.e., the materials of both the inner layer and the outer layers.
For this purpose, advantage is taken of the fact that although a magnetic flux of a relatively low frequency penetrates far into the coin, a magnetic flux of a relatively high frequency penetrates only the thin outer layer of the coin. Accordingly, a plural number of oscillators are provided which produce oscillation signals of relatively low and high frequencies, thereby allowing for the detection of the inner and outer materials of the coin. Specifically, oscillation coils of the oscillator circuits are arranged along a coin passage so as to cause the coin first to pass through a low-frequency magnetic flux and subsequently through a high-frequency magnetic flux as it is being conveyed.
In a second type of conventional coin sorting machine, as disclosed in Japanese Patent Laid-Open No. 3-180992, transmitting and receiving coils as well as an oscillator are provided. The transmitting coil is disposed opposite to the receiving coil with the coin passage lying therebetween, and the oscillator is arranged to alternatively apply currents of different frequencies to the transmitting coil. Like the first-described conventional coin sorting machine, on which it is an improvement, the second type of conventional coin sorting machine also takes advantage of the fact that as the frequency of the applied current increases, the penetration depth of the induced flux into the coin is reduced.
However, the aforedescribed conventional examples have the following problems.
In the first type of coin sorting machine, because a plurality of oscillation coils are arranged along the coin passage, during the time period from the instant an inserted coin passes the first oscillation coil until it passes the last oscillation coil, it is not possible to determine whether to accept another coin. It is also necessary to extend the coin passage in the direction of coin movement to accommodate the oscillation coils. Thus, the time required to make a decision after a coin is inserted is relatively long. As a result, it may occur that the time between the decision to accept a coin and the activation of an acceptance mechanism, such as an acceptance gate, is insufficient. To secure sufficient time, for such purpose, it thus may be necessary to extend the distance from the oscillation coil to the acceptance mechanism. Consequently, the overall coin sorting machine may become bulky.
In the second type of coin sorting machine, because a single pair of transmitting and receiving coils is utilized, it is not necessary to increase the length of the coin passage or to otherwise increase the overall size of the device. However, the oscillator which applies in succession currents of different frequencies to the transmitting coil is complex in structure and expensive.
Accordingly, it is an object of the present invention to provide an inexpensive, high-performance coin sorting machine whose size, and hence decision time, is reduced by minimizing the coin passage length, which is capable of handling more than one coin simultaneously, and in which a simple oscillation circuit is used to provide oscillation signals of different frequencies.